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NCDOT level of service software program for highway capacity manual planning applications : final report

Appendix A: 2000 HCM Facility A- 5
Methodology and Calculations
necessary because heavy vehicles require more space than passenger cars do. This factor
can be found using HCM Equation 23- 3.
1 ( 1) ( 1)
1
+ − + −
=
T T R R
HV P E P E
f ( 23- 3)
where
fHV = heavy- vehicle adjustment factor,
ET = passenger- car equivalent for trucks/ buses,
ER = passenger- car equivalent for recreational vehicles ( RVs),
PT = proportion of trucks/ buses in the traffic stream, and
PR = proportion of RVs in the traffic stream.
The passenger- car equivalent ( PCE) values for trucks, buses, and RVs represent
the number of passenger cars that would occupy the same amount of the freeway capacity
as one truck, bus, or RV. This factor accounts not only for the increased length of a
heavy- vehicle but also decreased heavy- vehicle performance due to the freeway grades.
When there is no grade 3 percent or greater for longer than 0.25 miles or no grade of 2- 3
percent for greater than 0.5 miles, Table A6 can be used to find the PCE. Otherwise, a
specific grade analysis must be performed.
Type of Terrain
Factor Level Rolling Mountainous
ET ( trucks and buses) 1.5 2.5 4.5
ER ( RVs) 1.2 2.0 4.0
When finding the PCE using the extended freeway segment analysis in Table A6,
the terrain must be classified as level, rolling, or mountainous. Level terrain is comprised
of short grades no greater than 2 percent, which allow heavy vehicles to travel at the
same speed as passenger cars. Rolling terrain causes heavy vehicles to operate at slower
speeds than passenger cars but without having to resort to crawling speed. Finally,
mountainous terrain causes heavy vehicles to operate at a very low rate of movement
( crawl speed). A truck’s crawl speed is the maximum speed that the truck can sustain on
an extended grade. Eventually, if any grade is long enough a truck will reach its crawl
speed for that grade.
For specific grade analysis, the percent of grade, length of grade, and proportion
of heavy vehicles must be considered. Table A7 and A8 can be used to evaluate upgrade
segments for trucks and RVs, respectively. For downgrades, if trucks do not have to shift
into a low gear, the segment may be treated as if it were level terrain; otherwise, Table
A9 can be used. For RVs, all downgrades may be treated as level terrain. For composite
grades, the average grade can be used if all subsections are less steep than 4 percent or
Table A6. Passenger- Car Equivalents on Extended Freeway Segments
( HCM Exhibit 23- 8)

Appendix A: 2000 HCM Facility A- 5
Methodology and Calculations
necessary because heavy vehicles require more space than passenger cars do. This factor
can be found using HCM Equation 23- 3.
1 ( 1) ( 1)
1
+ − + −
=
T T R R
HV P E P E
f ( 23- 3)
where
fHV = heavy- vehicle adjustment factor,
ET = passenger- car equivalent for trucks/ buses,
ER = passenger- car equivalent for recreational vehicles ( RVs),
PT = proportion of trucks/ buses in the traffic stream, and
PR = proportion of RVs in the traffic stream.
The passenger- car equivalent ( PCE) values for trucks, buses, and RVs represent
the number of passenger cars that would occupy the same amount of the freeway capacity
as one truck, bus, or RV. This factor accounts not only for the increased length of a
heavy- vehicle but also decreased heavy- vehicle performance due to the freeway grades.
When there is no grade 3 percent or greater for longer than 0.25 miles or no grade of 2- 3
percent for greater than 0.5 miles, Table A6 can be used to find the PCE. Otherwise, a
specific grade analysis must be performed.
Type of Terrain
Factor Level Rolling Mountainous
ET ( trucks and buses) 1.5 2.5 4.5
ER ( RVs) 1.2 2.0 4.0
When finding the PCE using the extended freeway segment analysis in Table A6,
the terrain must be classified as level, rolling, or mountainous. Level terrain is comprised
of short grades no greater than 2 percent, which allow heavy vehicles to travel at the
same speed as passenger cars. Rolling terrain causes heavy vehicles to operate at slower
speeds than passenger cars but without having to resort to crawling speed. Finally,
mountainous terrain causes heavy vehicles to operate at a very low rate of movement
( crawl speed). A truck’s crawl speed is the maximum speed that the truck can sustain on
an extended grade. Eventually, if any grade is long enough a truck will reach its crawl
speed for that grade.
For specific grade analysis, the percent of grade, length of grade, and proportion
of heavy vehicles must be considered. Table A7 and A8 can be used to evaluate upgrade
segments for trucks and RVs, respectively. For downgrades, if trucks do not have to shift
into a low gear, the segment may be treated as if it were level terrain; otherwise, Table
A9 can be used. For RVs, all downgrades may be treated as level terrain. For composite
grades, the average grade can be used if all subsections are less steep than 4 percent or
Table A6. Passenger- Car Equivalents on Extended Freeway Segments
( HCM Exhibit 23- 8)